William E. Doll

An Evaluation of Methods and Available Software for Seismic Refraction Tomography Analysis

Seismic refraction tomography is an alternative to conventional seismic refraction analysis methods. While the limitations and potential pitfalls of conventional refraction methods are well-known the same is not true for refraction tomography. As refraction tomography becomes more widely used, the need to know and understand its capabilities as well as its limitations becomes more critical. In this study we created eight representative models for use in evaluating three commercially available codes as well as refraction tomography in general. These models range from simple two-layer or dipping-layer problems to more complicated models designed to represent features of karst terrains. We demonstrate quantitatively and qualitatively that all three codes perform at a similar level, although each has strengths and weaknesses. Refraction tomography performs well in many situations where conventional methods fail, e.g., where lateral or vertical gradients compose a significant component of the velocity structure.

A workshop examination of shallow seismic reflection surveying

In September 1996 the Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) sponsored a research workshop in Berkeley, California, where approximately 20 participants analyzed the potential and limitations of near‐surface seismic‐reflection methods.

Airborne geophysical surveying for hazardous waste site characterization on the Oak Ridge Reservation, Tennessee

Airborne geophysical methods that were developed for mineral and petroleum exploration can, with some modification, be applied to environmental problems where large areas must be characterized. A helicopter survey that deployed magnetic, electromagnetic, and radiometric sensors carried out one of the first large‐scale airborne environmental surveys at a U.S. government facility at Oak Ridge, Tennessee in 1993–1994. The survey included testing of a new airborne electromagnetic system designed specifically for environmental applications and for controlled field tests of magnetic systems. Helicopter‐borne magnetic measurements were capable of discriminating groups of as few as ten metallic 208-liter (55-gallon) storage drums under representative field conditions. Magnetic and electromagnetic sensors were able to distinguish groups of metal‐filled waste disposal trenches within disposal sites, but were unable to resolve individual trenches. Electromagnetic data proved to be the most effective airborne techniq...

A noninvasive shallow seismic source comparison on the Oak Ridge Reservation, Tennessee

Nine seismic sources were compared in preparation for production seismic reflection profiling to about 500 m depth at a hazardous waste site on the Oak Ridge Reservation, Tennessee. Test data provided a 125-shotpoint common midpoint profile and a walkaway vertical seismic profile for each source. Sources tested included five swept sources and four impulsive sources. Eight of the sources were noninvasive, in that they would not penetrate the ground surface. When spectral whitening methods were applied to the data, the IVI Minivib provided the best image of the subsurface on the basis of continuity and clarity of reflections. Without prewhitening, the performance of the IVI Minivib and the Bison elastic wave generator were similar.

Seismic reflection evidence for the evolution of a transcurrent fault system: The Norumbega fault zone, Maine

A seismic reflection profile in east-central Maine reveals a steeply dipping fault zone that terminates in diffraction hyperbolae that are associated with an offset of the Moho. When combined with data from geologic mapping, the seismic data imply Mesozoic or post-Mesozoic dip-slip reactivation of the Norumbega fault zone, hitherto interpreted as a locus of mid- to late Paleozoic dextral offset. This finding highlights the value of seismic reflection data in determining the history of fault displacement and warns of the complexities that arise in attempts to seismically characterize faults in polygenic tectonic regimes.

Recent advances in airborne survey technology yield performance approaching ground-based surveys

Airborne magnetic and electromagnetic systems have been very effective over the years for mineral prospecting and in support of petroleum exploration. More recently, these towed-bird systems, operating at sensor altitudes of 30–50 m, have supported environmental investigations. The towed-bird systems can provide regional data for site investigations, such as locating or delimiting the boundaries of waste areas, identifying geologic contacts that influence environmental issues, or mapping saline intrusion. However, these conventional systems cannot provide the resolution required in many environmental and engineering problems because the distance between sensors and target objects is too great. Ground-based surveys are often suitable for addressing these problems but, for many sites, the area can be too large to be expediently addressed with surface geophysics. Contamination of government land with unexploded ordnance (UXO) is one such large-scale problem. As military bases are being closed and returned to civilian use, the removal of UXO has become a critical problem. By one estimate, about 11 million acres within the United States, roughly the sum of the areas of the states of New Hampshire and Vermont, are contaminated with UXO. This includes Department of Defense (DOD) sites, Department of Energy (DOE) sites, Native American sites, and National Park lands. At some DOD sites, large areas are currently off limits for training pending removal of UXO. In other parts of the world, UXO threatens peoples lives every day. In the 1990s, two coauthors of this article, Holladay and Gamey, began to address the UXO problem while working at Aerodat in Toronto. They devised a three-magnetometer system, the HM-3, in which the sensors were mounted in booms attached directly to the helicopter. This architecture provided an opportunity for the pilot to safely fly much closer to the surface. During subsequent joint projects with Oak Ridge National Laboratory (ORNL) researchers, both …

Field tests of an experimental helicopter time-domain electromagnetic system for unexploded ordnance detection

Field trials of a low-flying time-domain helicopter electromagnetic system designed for detection of unexploded ordnance have yielded positive and encouraging results. The system is able to detect ordnance as small as 60-mm rounds at 1-m sensor height. We examined several transmitter and receiver configurations. Small loop receivers gave superior signal-to-noise ratios in comparison to larger receiver loops at low heights. Base frequencies of 90 Hz and 270 Hz were less affected than other base frequencies by noise produced by proximity to the helicopter and by vibration of the support structure. For small ordnance, a two-lobed, antisymmetric transmitter loop geometry produced a modest signal-to-noise enhancement compared with a large single rectangular loop, presumably because the antisymmetric transmitter produces smaller eddy currents in the helicopter body, thereby reducing this source of noise. In most cases, differencing of vertically offset receivers did not substantially improve signal-to-noise ratios at very low sensor altitudes. Signal attenuation from transmitter to target and from target to receiver causes signals from smaller ordnance to quickly become indistinguishable from geological background variations, so that above a sensor height of about 3 m only large ordnance items (e.g., bombs and large caliber artillery rounds) were consistently detected.

Geophysical Character of Buried Sinkholes on the Oak Ridge Reservation, Tennessee (Presented at SAGEEP95 in Orlando, FL)

Sinkholes are important near‐surface indicators of active karst features at depth, such as cavities, conduits and solutionally‐enlarged fractures. This study tests the hypothesis that ground‐penetrating radar (GPR) and other geophysical methods may be used to identify filled sinkholes as a means of inferring the existence of these deeper hydraulically‐active karst features. Geophysical surveys were made at two sites in Bear Creek Valley on the Oak Ridge Reservation in eastern Tennessee. GPR profiles (25 and 50 MHz) made along the northeast edge of Bear Creek Valley showed a troughlike pattern of radar reflections outlining a possible filled sinkhole. This feature is at least 45 m wide, 3.5 to 5 m deep, and appears on GPR profiles acquired at a variety of antenna separations and along profiles made in different directions. Its dimensions are consistent with dimensions of other mapped sinkholes in the area. GPR, electromagnetic conductivity surveys, resistivity soundings and gravity models suggest this feat...

Airborne vertical magnetic gradient for near-surface applications

Many specialized applications in near-surface geophysics require greater spatial and amplitude sensitivity than conventional geophysical systems can provide. Recently developed boom-mounted airborne magnetometer and electromagnetic systems are designed to map unexploded ordnance or other small metallic objects over large tracts of government land. These systems operate at altitudes of 1.5–2.0 m to detect ferrous objects with a mass as small as 2 kg. In this low-altitude environment, factors controlling the applicability of a vertical gradient configuration must be reassessed. Our results demonstrate the superiority of measured vertical gradient over calculated gradient (from total field configurations) for data acquired at these low altitudes. We believe the effectiveness is related to reduction of secondary gridding effects associated with several types of positioning errors, and reduction of correlated rotor and compensation noise.

Evaluation of refraction tomography codes for near‐surface applications

What are the advantages of tomographic methods over delay-time methods? Until recently seismic refraction analysis has been limited to generalized reciprocal, delay-time, or other techniques that require simplifying assumptions such as constant velocity layers and lateral homogeneity within (e.g. Lankston, 1990). The faster and more powerful computers available today have led to the development of various seismic tomography routines. Tomography methods do not make the limited assumptions mentioned above, although each has its own limitations. We compare the performance of three commercially available tomography codes and a delay-time code for representative near-surface models. We use a data set from a site on the Oak Ridge Reservation in East Tennessee (Watson et al., in press, Doll et al., 2002) to represent a typical situation. Results from the synthetic data for each tomography code are compared with results from conventional delay-time analysis. We examine the strengths and weaknesses of these tomographic routines and identify possible artifacts from the inversion. • What are the limitations and strengths of each code studied? • What kind of artifacts or other inaccuracies occur during tomographic analysis? • How does acquisition geometry affect the results of tomography?